Reliable assessment of increased myocardial extracellular volume, often due to diffuse interstitial fibrosis, is of significant clinical interest due to its ubiquitous presence in many cardiac diseases. Recent studies have shown altered native myocardial T1 values in patients with a wide range of diseases including patients with ST-segment elevation myocardial infarction (STEMI) and non-STEMI [1] and hypertrophic and dilated cardiomyopathy [2], as well as infiltrative diseases such as amyloidosis [3, 4], iron overload [5], and Anderson-Fabry disease [6-8]. T1 mapping with gadolinium contrast can also be used to estimate the extracellular volume fraction (ECV), a physiologically relevant parameter that increases with diffuse interstitial fibrosis and infiltrative diseases, and abnormal ECV values have been reported across a wide spectrum of cardiac diseases [9-11].
Correlations of abnormal T1 values with other biomarkers of myocardial remodeling have also been reported in asymptomatic patients with diabetes [12], aortic stenosis [13], and hypertension with left ventricular hypertrophy [14], suggesting T1 may also be a sensitive marker of early pre-clinical remodeling.
Reliable ECV quantification requires techniques with both high accuracy and precision. Accurate T1 mapping techniques without systematic confounders are intuitively desirable because they may be more specific to T1 changes associated with changes in ECV. While the MOdified Look-Locker Inversion recovery (MOLLI) technique [15, 16] has gained widespread adoption, it is sensitive to factors such as T2[17], magnetization transfer [18], and off-resonance [19], and changes in any of these confounders result in changes in measured T1 values [20]. Saturation-recovery based sequence such SAturation-recovery single-SHot Acquisition (SASHA) [21], Saturation Method using Adaptive Recovery Times (SMART1Map) [22], and SAturation Pulse Prepared Heart rate independent Inversion-REcovery sequence (SAPPHIRE) [23] are more robust to these confounders, but their adoption has been limited by poorer precision, which results from reduced dynamic range and signal-to-noise compared to the inversion-recovery based MOLLI sequence [20, 24]. Higher precision techniques with less variability are needed to reliably detect subtle T1 changes in individual patients and to better identify focal T1 abnormalities.
Free-breathing T1 mapping techniques can potentially increase precision compared to breath-hold techniques by acquiring more images over a longer duration, thus reducing uncertainty in calculated T1 values. Free-breathing approaches also extend the utility of T1 mapping to patients who are unable to adequately hold their breath, such as those with shortness of breath or heart failure. One common approach for addressing respiratory motion during free-breathing T1 acquisitions is to use respiratory navigator triggering, such as in the high-resolution Accelerated and Navigator-Gated Look-Locker Imaging for cardiac T1 Estimation (ANGIE) T1 mapping sequence [25]. The position of the diaphragm is monitored using a separate acquisition and imaging is performed within a small window of respiratory phase. However, cardiac imaging with respiratory navigation may still have considerable residual motion because the heart and lung do not always move in perfect unison. Additionally, respiratory navigators can be challenging in routine clinical practice, as clinical patients often have irregular respiratory patterns that reduce navigator gating efficiency.
An alternative approach to free-breathing imaging is to continuously acquire images in all respiratory phases and use image registration to align a subset of images. This approach has been successfully applied to late gadolinium enhancement [26] and T*2 imaging [27], but direct image registration of SASHA's saturation recovery images is challenging due to poor tissue-blood contrast. While T1 mapping with saturation recovery is one particular example of a parametric mapping technique with poor, or varying blood-tissue contrast, parametric mapping of other parameters such as diffusion, magnetization transfer, T2, etc. may also benefit from approaches aimed at improving non-rigid registration in the setting of low-contrast images.
It is with respect to these and other considerations that the various aspects of the disclosed technology as described below are presented.